Capacitors for use in electronic apparatus such as portable telephones and personal computers are required to be small in size and large in capacitance. Of those capacitors, a tantalum capacitor is preferably used, because the capacitance is large, not in proportion to the size, and the tantalum capacitor also has good characteristics. The tantalum capacitor usually employs a sintered body of a tantalum powder as an anode. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the weight of the sintered body, or to use a sintered body having an increased surface area obtained by pulverizing the tantalum powder.
The increase in weight of the sintered body inevitably enlarges the shape, so that the requirement for a small-sized capacitor is not satisfied. On the other hand, when the tantalum powder is finely pulverized to increase the specific surface area, the pore size in the tantalum sintered body decreases, and the number of closed pores increases during the sintering step. The result is that the sintered body cannot be easily impregnated with a cathode agent in the subsequent step.
One approach to solve these problems is a capacitor using a sintered body of a powder material which has a greater dielectric constant than the tantalum powder. One powder material which has such a greater dielectric constant is a niobium powder.
Japanese Laid-Open Patent Application No. 55-157226 discloses a method for producing a sintered element for a capacitor. This method comprises the steps of subjecting a niobium powder ranging from an agglomerate to fine particles with a particle diameter of 2.0 μm or less to pressure molding and sintering, finely cutting the molded sintered body, connecting a lead portion to the finely cut particles of the sintered body, and thereafter sintering the connected body. However, the above-mentioned application does not describe the detailed characteristics of the obtained capacitor.
U.S. Pat. No. 4,084,965 discloses a capacitor using a sintered body of a niobium powder with a particle size of 5.1 μm obtained from a niobium ingot through hydrogenation and pulverizing. The capacitor disclosed has, however, a high leakage current value (hereinafter referred to as an LC value), and therefore the serviceability is poor.
As disclosed in Japanese Laid-Open Patent Application No. 10-242004, the LC value is improved by partially nitriding a niobium powder. However, when a capacitor having a large capacitance is produced using a sintered body of a niobium powder with a smaller particle diameter, the LC value of the obtained capacitor may become exceptionally high.
U.S. Pat. No. 6,051,044 discloses a niobium powder which has a particular BET specific surface area and, contains nitrogen in a particular amount. A method for decreasing the leakage current is also disclosed. However, there is no disclosure nor suggestion concerning a niobium powder containing another element which can form an alloy with niobium. Furthermore, this patent does not disclose nor suggest the heat resistance necessary for capacitors in soldering and the like or stability of LC value against thermal history.